Body panels for automotive vehicles are currently being manufactured using a hot stretch forming process applied to certain superplastically formable magnesium-containing aluminum alloy sheet stock. Examples of suitable forming practices are disclosed in U.S. Pat. No. 6,253,588 titled Quick Plastic Forming of Aluminum Alloy Sheet Metal; U.S. Pat. No. 6,880,377 titled Method for Double Action Gas Pressure Forming Sheet Metal; and U.S. Pat. No. 6,886,383 titled Method for Stretch Forming Sheet Metal by Pressing and the Application of Gas Pressure. Each of these patents is assigned to the assignee of this invention.
At the present time, the sheet stock that is used as a starting material is a fine grain microstructure, aluminum alloy 5083 having superplastic forming properties. Initially, the alloy is cast into a slab of a suitable thickness and subjected to a homogenizing heat treatment. The slab is then gradually reduced in thickness by a series of hot rolling operations to a strip in the range of twenty to forty millimeters depending somewhat on the goal for the final thickness of the sheet. The strip is cold rolled, usually in stages with interposed anneals, to a final sheet thickness in the range of about one to three or four millimeters. The result of such thermomechanical processing is a coil of smooth surface, aluminum sheet stock, the microstructure of which has been severely strained.
If this aluminum sheet stock is ultimately to be formed into automotive vehicle external body panels, or the like, the smooth surface of the cold rolled sheet is very important. The cold rolled surface must be free of visual defects and that surface quality must be retained in subsequent heating, forming and finishing steps so that the visible surface of the final part is commercially acceptable. In automotive manufacturing such a surface is called a “Class A surface.”
U.S. Pat. No. 6,890,394, Heating of Metal Alloy Sheet by Thermal Conduction, assigned to the assignee of this invention, discloses practices for conductive heating of cold rolled sheets of superplastic-type metal alloys between heated platens to recrystallize the microstructure of the sheets for hot stretch forming into automotive body panels or other articles of manufacture.
The process starts with a metal sheet blank of thickness and outline for hot stretch forming of a part. In a preferred embodiment, a blank of highly strained, cold rolled AA5083 composition is placed one-side-down on a flat surface of an electrical resistance heated platen and an opposing heated platen is brought into closely spaced relation to the topside of the blank. The blank is heated by conductive heat transfer from the hot platens to rapidly recrystallize its highly strained microstructure to a softened fine-grain structure. The heating also raises the temperature of the sheet material to a suitable stretch forming temperature. The platens are separated and the hot softened sheet moved to a nearby hot stretch forming apparatus with one or more forming tool surfaces against which the sheet is pressed into a body panel or other article. Indeed, the time requirements of the thermally conductive pre-heating step on the blank and the hot stretch forming of the blank are quite similar, facilitating efficient manufacturing. The disclosed conductive annealing method can be practiced so that the formed articles retain a surface finish like the cold rolled starting material.
The full disclosure of the '394 patent is incorporated into this specification by reference.
Annealed (recrystallized) blanks formed by the '394 process typically retain the high quality visible surfaces of the cold-rolled starting material. However, it has been found that some formed parts, depending upon their shape and amount of deformation, display spaced visible bands or stripes on low-strain areas of the article. The stripes are visible after stoning the surface of the formed part or after painting the part. The surface effect on formed articles has been termed “zebra stripes” because of the generally parallel, spaced pattern of the stripes. The stripes do not appear on all parts, and when they appear they are most commonly found in an area of a formed sheet that has experienced little or no deformation. The stripes are not associated with pronounced surface contours on the article and can be removed by abrasive sanding or other surface smoothing processes. However, it is preferable to avoid formation of the stripes.
The presence of zebra stripes has been attributed to the conductive heating and subsequent forming of some blanks. It is an object of this invention to provide a modified conductive heating practice for the blank sheets to eliminate the presence of zebra stripes on surfaces of formed parts.